Such an apparatus is disclosed in the U.S. Pat. No. 4,736,399.
When an X-ray image of an object is produced by detection of the X-rays which are locally intensity-modulated by the object by means of an X-ray image intensifier tube, geometrical distortions are produced in the image. An input screen of the X-ray image intensifier tube is usually curved, which results in a barrel or pin-cushion distortion of the X-ray image to be displayed. The television pick-up device coupled to an output window of the X-ray image intensifier tube also introduces image distortion, as a result of which distances in the ultimate X-ray image do not correspond to the actual dimensions. This is particulary disturbing if quantative data on relative positions of objects must be derived from the X-ray image and, in tomographic imaging, disturbs an accurate image reconstruction. To obviate these disadvantageous effects, the X-ray image is subjected to a geometrical correction, for which purpose the video signal received from the television pick-up device is digitized and stored in a memory of an image correction device. Thereafter image values I(u,v) of the digitized video signal are converted into image values I(x,y) of a corrected video signal. Therein u and v are the coordinates of an image element in the distorted X-ray image, and x and y the coordinates associated with this image element in the corrected X-ray image. For each pair of coordinates (x,y) of the corrected X-ray image the image value I(u,v) associated therewith and stored at an address corresponding to coordinates (u,v), is stored again at an address corresponding to coordinates (x,y). Herein the relation between addresses corresponding to (x,y) and (u,v) are recorded as a vector in a RAM. The corrected image values I(x,y) are applied to a television monitor via a DA converter. Such a method of image correction has the disadvantage that before correction can be effected, the entire X-ray image must first be stored in a memory. Consequently the time required for image correction is relatively long. Particularly during operations while using fluoroscopy it is desirable to have the corrected X-ray images available as quickly as possible.